Production of fluoborate compounds



Patented Dec. 5, 1939 PATENT OFFICE,

- 2,182,511 I PRODUCTION or nuonom'm COMPOUNDS Harold W. Heiser, EastSt. Louis, 111., assignor to, Aluminum Company of- America, Pittsburgh,

Pm, a corporation of Pennsylvania No Drawing. Application August a,1939',

Serial 110.289.812 v 9 Claims. (c1. 23-59) This application is acontinuation-in-part of my copending application Serial No. 144,412,flied May 24, 1937.

The invention relates to the extraction of fluorine values fromfluorspar ores, and is particularly concerned with a method for theproduction of hydrofiuoboric acid and the soluble fluoborate salts.

The commercial processes that have been gen- 10 erally used for therecovery of the fluorine values of fluorspar ores involve thedecomposition of the fluorspar at elevated temperatures in the presenceof strong mineral acids such as sulfuric acid. The fluorine content ofthe ore is liberated in gaseous form as hydrofluoric acid containinghydrofluosilicic acid as an impurity in an amount dependent upon theamount of silica present in the fluorspar ore used, and the residue is ahighly calcined calcium compound of the mineral acid used, such asanhydrous calcium sulfate. The

recovery of the hydrofluoric acid is generally ac complished bycondensation of the gas in water.

The handling of this gaseous product is, however,

quite dimcult and expensive, and the elevated temperatures used requireequipment highly resistant to acid corrosion. Furthermore, as a resultof the high temperatures used, the by-product calcium salt produced isgenerally in suchform as to require considerable processing to 39 makeitcommercially useful. This is particularly so when a by-product calciumsulfate is produced in anhydrous form.' In order to make the anhydrouscalcium sulfate suitable for use in the plaster industry, which is themost obvi- ,35 ous outlet for this by-product, it is necessary that adiflicult and expensive hydration step'and a subsequent partialdehydration 'stepbe performed.

These processes require the use of the so-called o acid-grade fluorsparores to keep the contamination of the product by silica to an acceptableminimum. Acid grade fluorspar is generally specified to contain lessthan 1 per cent of silica, and such material occurs only sparingly inna- 5 ture. Natural fluorspar ores containing higher percentages ofsilica must be subjected to elaborate and expensive purificationprocesses to reduce the quantity of silica present before the materialis acceptable for use in the previously 50 used acid-recovery processes.These factors have contributed a large element to the relatively highcost of fluorine-containing compounds which are required to besubstantially free from silica.

It is an object of this invention to provide an improved method for theextraction of fluorine values from fluorspar ores in which no gaseousfluorine compounds are produced and the dimculties incident to thehandling of such compounds are thus eliminated. Another object of thisinvention is to provide a method for the 5 extraction of the fluorinecontent from fluorspar ores, which method is carried out at moderatetemperatures, and in which the by-product calcium sulfate is produced inthe form of hydrated calcium sulfate and is readily rendered utilizablefor commercial applications. A further object of my invention is toprovide a method for the production of substantially silica-freehydrofluoboric acid and soluble fluoborate salts by the decomposition offluorspar containing silica as an impurity at moderate temperatures,with the simultaneous production of hydrated calcium sulfate.

My invention is predicated upon the discovery that by digestion ofcalcium fluoride or other 20 alkaline earth fluorides with an aqueoussolu- 'tion' of sulfuric acid conta ning boric acid or a boric acidcompound (by which term is meant a metallic salt of boric acid such ascalcium or sodium borate) a-solution of hydrofluoboric acid 25 or asoluble fluoborate salt substantially free from silica and hydratedcalcium'sulfate may be 'produced. By this method I have found thatsubstantially complete recovery of the fluorine content of a fluorsparore may be effected without the necessity of using a gas-tight apparatusor apparatus which is highly resistant to corrosion by hydrofluoricacid. Furthermore, I have found that a hydrofluoboric acid or fluoboratesolution which is substantially free from silica may be produced by thismethod even though a lowgrade fluorspar containing a relatively highpercentage of silica is used as the starting material, and a highlyvaluable by-product of hydrated calcium sulfate may be obtained.

In carrying out the process of my invention in the preferred manner.fluorspar or calcium fluoride is first ground to a suitably smallparticle size. Either relatively pure (acid-grade) spar or low-gradespar containing substantial 5 amounts of calcite or silica or both maybe used. Calcite in the fluorspar ore does not interfere with thepractice of the process except insofar as it uses up sulfuric acid andis converted to hydrated calcium sulfate. As above noted, the presenceof silica in the ore is not detrimental to the hydrofluoboric acid orsoluble fiuoborate produced, since the silica has been found to besubstantially insoluble in the digest liquor at the moderatetemperatures used, presumably due a to the presence of the boric acid orboric acid compound, and particularly when the boric acid or boric acidcompoundis present in execs of I the stoichiometric amount required tocombine 5 with all of the fluorine released from the calcium fluoride insolution. For example, I have been able produce fluoborate solutionscontaining substantially no silica from fluorspar ores containing ashigh as 15 per cent silica. As the silica is insoluble in the digestliquor at the temperatures used, it settles out of the solution with thecalcium sulfate which is precipitated.

In the practice of my invention, ground fluorspar is mixed in a suitablevessel with an aqueous sulfuric acid solution containing boric acid or asalt capable of yielding boric acid in acid solution. For this purpose,I have found it most desirable to use-a 10 M per cent solution ofsulfuric acid in'water in an amount sumcient to 20 provide thetheoretical quantity or somewhat less than the theoretical quantity ofsulfuric acid required to decompose all the calcium fluoride present inthe spar. While an excess of sulfuric acid may be used if desired, theexcess over that required for reacting with the calcium fluoride willremain in the hydrofiuoboric acid or fluoborate product. In order toobtain a maximum yield and a product substantially free from silica, theboric acid compound is preferably somewhat in excess of the amountrequired to combine with all of the fluorine in the fiuorspar, accordingto the equation The finely, ground fluorspar and the aqueous solution ofsulfuric acid and boric acid compound are then preferably heated to atemperature of 70 to 100 C. for a period of 1 to 3 hours, within whichtime substantially complete reaction can be efiected. Shorter periods ofdigestion at higher temperatures, or longer periods of digestion attemperatures as low as room temperature may be used if desired, but forcommercial practice I have found it most convenient to digest themixture at temperatures below the boiling point for a period sufficientto produce substantially complete reaction.

The digestion of the fiuorspar with the sulfuric acid solutioncontaining the boric acid compound may be carried out in two stages toproduce substantial economies by the complete extraction of the fluorinefrom the fluorspar and the production of a hydrofiuoboric acid or asoluble fluoborate substantially free from sulfuric acid. In carryingout the process in this manner, an excess of fiuorspar is first treatedwith sulfuric acid solution containing the boric acid compound.

The hydrofiuoboric acid solution thus produced is separated from thehydrated calcium sulfate sludge containing the excess fluorspar and theinsoluble impurities in the fiuorspar, including silica. The sludge isthen digested with an excess of a fresh solution of sulfuric acidcontaining the boric acid compound to obtain complete extraction of thefluorine from the fluorspar remaining in the sludge. The solution thusproduced is then used in treating a further quantity of fluorspar ore inexcess. By a continuous twostage digestion process operated in thismanner, it is possible to effect substantially complete recovery of thefiuorine. Yields .as high as 99.8 per cent of the fluorine present inthe fiuorspar have been obtainedin this manner. It is to be understoodthat if desired. the sultained from the digestion of the fiuorspar willfuricacid may be replaced in part by an equivalent sulfate salt in thedigestion liquor, or the source of the boric acid may be wholly or inpart a salt .of boric acid. When the sulfuric acid is replaced in partby an equivalent sulfate salt. 5 or the boric acid is replaced in wholeor in part by a salt of boric acid, the salt selected should be onewhich will give a compound of hydrofluoboric acid which is soluble inthe solution under the digestion conditions used, and by the expressionsoluble salts of hydrofiuoboric acid is meant those salts of the acidwhich are substantially soluble in the solution under the conditionsdiscussed herein. In such case, the solution obcontain an equivalentproportion of the corresponding fiuoborate salt.

When a two-stage digestion of fiuorspar is carried out and an excess ofa relatively strong sulfuric acid solution, such as a 20 per centsulfuric acid solution containing a boricacid compound and a substantialamount of sodium sulfate, is

used in the second digestion stage (in which the sludge containinghydrated calcium sulfate and fluorspar is digested), not only is thefluorine extracted from the sludge, but the hydrated calcium sulfate isconverted to finely divided anhydrous calcium sulfate. Thus, by suitableregulation of the conditions of operation and the composition of thedigest liquor, it is possible to produce either hydrated or anhydrouscalcium sulfate as a by-product of the method of my invention.

As an example of the production of a hydrofiuobpric acid free fromsulfuric acid and silica in a single stage process, 252 grams offluorspar containing approximately 83.2 per cent CaFz, 12.7 per centS102, and 3.2 per cent CaCO: was digested in a 1000 c. c..aqueoussolution containing 260-grams H2804 and grams HaBO: at a temperature of90 C. for a period of 2 hours. The hot slurry was filtered and theliquor allowed to cool. It was found that the clear cooled liquorcontained a solution of 108 grams per liter of substantially silica-freehydrofluoboric acid. The calcium sulfate residue which was removed fromthe filtrate contained'all of the silica thatwas present in the rawfluorspar, while the fluoride content of the residue showed that 92 percent of the total fluorine had been extracted from the originalfiuorspar. As the fiuorspar digested was in an amount equal to per centof the stoichiometrical quantity required, the maximum amount offluorine which could have been recovered would have been about 95 percent.

Higher fluorine recovery may be obtained by the use of an excess ofsulfuric and boric acids. For example, finely ground fiuorspar wasdigested with an aqueous 10 per cent solution of H2804 containing boricacid in an amount equivalent to about per cent of the theoreticalrequirement. The digestion was carried out for a period of two hours. ata temperature of 90 to 93 C. The hydrofiuoboric acid liquor thus formedwas filtered off from the solid hydrated calcium sulfate. The liquorcontained 97.6 per cent of the fluorine originally present in thefiuorspar.

Still higher fluorine recovery values may be obtained by the use of atwo-stage digestion process, in whicha large excess of fiuorspar is usedin the first stage and, in the second stage, a large excess of sulfuricand boric acids is used to insure -extraction of the fluorine from theCaSOr-Cal":

residue resulting from the first stage, the liquor from the second stagedigestion being subse- '75 i HBF4.

ple of thepreparation of hydrofluoboric acid by this latter method, a1000 c. 0. aqueous solution containing 264 grams H2804 and 100 gramsH3303 was used to digest 360 grams of fluorspar containing approximately83.2 per cent, CaFz, 12.7 per cent SiOz, and 3.2 per cent CaC03. The hotslurry was filtered and the liquor allowed to cool. An analysis of theclear cooled liquor showed that it was a solution containing 114 gramsper liter The CaFz-CaSO; residue was then redigested at 90 C. for 2hours in 1000 cubic centimeters of an aqueous solution containing 264grams H2804 and 100 grams 1131303. An analysis of the residuefrom thisdigestion showed that substantially all of the silica in the originalfluorspar was present in the calcium sulfate residue,rand a fluorineextraction of over 99 per cent was obtained.

I claim:

1. A method for the production of a material selected from the groupconsisting of hydrofluoboric acid and its salts which comprisesdigesting fluorsparwith an aqueous solution of sulfuric'acid and amaterial selected from the group consisting of boric acid and boric acidsalts,

-2.'A method for the production of a material selected from the groupconsisting of hydrofluoboric acid and its salts which comprisesdigesting fluorspar with an aqueous solution of sulfuric acid and amaterial selected from the group consisting of boric acid and boric acidsalts at a temperature below the boiling point.

3. A method for the production of a material selected from the groupconsisting of hydrofluoboric acid and its salts which comprisesdigesting fluorspar with an aqueous solution of sulfuric acid and amaterial selected from the group consisting of boric acid and boric acidsalts at temperatures of to C. for a period of 1 to 3 hours.

4. A method for the extraction of fluorine values from fluorspar orescomprising digesting the fluorspar with an aqueous solution of sulfuricacid containing a material selected from the group consisting of boricacid and boric acid salts.

5. A method for the extraction of fluorine .values from fluorspar orescomprising digesting the fluorspar with an aqueous solution of sulfuricacid containing a material selected from the group consisting of boricacid and boric acldsalts at temperatures of 70 to 100 0.101 a period of1 to 3 hours.

quently treated with'an excess of fluorspar to produce morehydrofluoboric acid. As anexam- 6. A method for the production of amaterial selected from the group consisting of hydrofluoboric acid andits salts, comprising digesting an excess of fluorspar with an aqueoussolution of sulfuric acid containing a material selected from the groupconsisting of boric acid and boric acid salts, separating the solutionthus formed from the sludge containing hydrated calcium sulfate and theexcess fluorspar, digesting the sludge with a further quantity of anaqueous solution of sulfuric acid containing a material selected fromthe group consisting of boricacid and boric acid salts in excess of theamount required to-extract the fluorine content of the sludge,separating the solution thus formed, and digesting a fresh quantity offluorspar in excess with said solution.

7. A method for the recovery of fluorine values substantially free fromsilica from fluorspar containing silica, comprising digesting fluorsparwith an aqueous solution of sulfuric acid containing a material selectedfrom the group -consisting of boric acid and boric acid salts to form asolution of hydrofluoboric acid and its soluble salts, and precipitatingcalcium sulfate together with the insoluble impuritiespresent in thefluorspar.

8. A method for the recovery of fluorine values substantially free fromsilica from fluorspar containing silica, comprising digesting fluorsparwith an aqueous solution of sulfuric acid containing a' materialselected from the group consisting of boric acid and boric acid salts toform a solution of hydrofluoboric acid and its soluble salts, and

separating the calcium sulfate formed together with the insolubleimpurities present in the fluorspar from the solution of hydrofluoboricacid and its salts.

9. A method for the recovery of fluorine values substantially free fromsilica from iiuorspar containing silica, comprising digesting fluorsparwith an aqueous solution of sulfuric acid containing a material selectedfrom the group consisting of boric acid and boric acid salts to form. asolution of hydrofiuoboric acid and its soluble salts, said materialbeing present in an amount in excess of the stoichiometric amountrequired to combine with all of the fluorine recovered, and separatingthe calcium sulfate formed together with the insoluble impuritiespresentin the fluorspar from the solutionof hydrofluoboric acid and itssalts.

HAROLD w. HEISER.

